This invention relates to cooling means for Roots-type superchargers or rotary blowers. More specifically, the present invention relates to means for cooling the supercharger to enable the supercharger to produce higher boost pressures. The present invention may be employed as a supercharger for internal combustion engines and for other purposes such as an air or gas compressor.
A major strategy being employed for improving automobile fuel economy is engine downsizing, or reducing the displacement and number of cylinders of the engine. Supercharging enables a small displacement engines to achieve the same power output as a larger displacement naturally aspirated engine. The degree of engine downsizing that can be attained with supercharging, and therefore the degree of fuel economy improvement, depends on the boost pressure that the supercharger can deliver. The boost pressure that a supercharger can deliver is currently limited by overheating of the supercharger. Supercharger cooling means are needed for increasing supercharger boost pressure and improving automobile fuel economy through engine downsizing.
Roots-type superchargers are shown in U.S. Pat. Nos. 4,768,934 and 5,078,583, and United States Patent Application Pub. Nos. US2011/0058974 A1 and US2014/0193285 A1. These superchargers use helical rotors for improved efficiency and reduced noise.
Referring now to U.S. Pat. No. 4,768,935, Raymond Soeters shows in FIGS. 4 and 5 backflow slots (40) and (42) for reducing supercharger noise by permitting back flow of pressurized air from the supercharger outlet port (38) directly into the back flow slots (40) and (42) and into the cylindrical chambers (32) and (34). The backflow slots reduce noise but have no reported cooling benefits.
Referring now to U.S. Pat. No. 5,078,583, Keith Hampton shows similar back flow slots (44) and (46) in FIG. 2. In FIG. 4 it can be seen that outlet port opening (26) and back flow slots (44) and (46) are directly joined through a common manifold or recess (42), as described in column 4 lines 37 through 56. The backflow air is generally hot because it has been compressed and because of its turbulent recirculation flow path. The backflow slots reduce noise but have no reported cooling benefits.
US Patent Application Publication Nos. US2011/0058974 A1 and US2014/0193285 A1 show helical rotors that provide for improve flow aerodynamics and improved supercharger efficiency. Referring to FIG. 7 in both of these publications, these superchargers have a blow hole or back flow port (51) that provides for internal backflow of compressed air, with external back flow slots no longer shown. It is the intent of the blow hole to minimize supercharger noise by allowing the low pressure intake air between rotor lobes and the housing to increase in pressure or more closely equalize with the outlet air pressure prior to the transition volume opening to the outlet port (19). The backflow air is generally hot because it has been compressed and because of its turbulent recirculation flow path through the blow hole. The blow hole reportedly reduces noise but does not provide cooling benefits.
A problem with prior art superchargers is that their working pressure or pressure ratio capability is limited by overheating. The working pressure can be increased by using large clearances between the rotors and housings to accommodate thermal expansion, but the large clearances result in increased leakage and lower supercharger efficiency.